RMS corresponds to the continuous power the the unit can put out and peak is how much power it can put out for very short peaks like a loud kick drum or guitar chord.
The cassette deck in my 2002 stock JBL system died and I used that for an XM interface. Until I found out about the inline antenna interface for the XM Roady, I was looking to replace my head unit. I had pretty much decided on the Alpine CDA-9883 head unit. All of my aftermarket units have been Alpine (since 1984) and have always been pleased with them.
That unit lists at 199 at crutchfield
http://www.crutchfield.com/S-cdfQcV3vnl ... DA9883#Tab
and has three sets of preamp outputs along with a 4x18 rms/50 watt peak amp built in. One of the preamp outputs is for a subwoofer and the unit has built in filters that you can set so that the deep base doesn't go to the regular speakers once you install the subs and amp.
Just be careful, if you have the stock speakers that amount of power can blow them. So until you replace the speakers, keep a close ear out for speaker distortion, which means you may be pushing them to far and almost to the point where they MAY blow.
I don't know what sound level you are looking for, but if you add a 100watt subwoofer to the above, that setup should suffice for almost anyone volume wise. I don't know if you have a coupe or convertible, but with the top down cruising on the interstate the 300 watts (4 x 50 peak, 1 x 100 subwoofer) of the factory JBL system is plenty loud for me listening to old school punk/ new wave.
HTH,
Len